High-voltage insulation and insulated high-voltage apparatus

ABSTRACT

An admixture of (1) from 55 to 70 percent by weight of hydrated alumina and (2) from 30 to 45 percent by weight of a petroleum oil extended ethylene-propylene-diene terpolymer, containing from 20 to 100 phr. of oil, is molded about electrical apparatus to provide insulation for components thereof. The hydrated alumina is advantageously finely divided so that a major portion or substantially all particles are less than 2 microns. The terpolymer is derived from the reaction of an admixture of (I) from about 85 to 99 molar percent of a mixture of (a) about 3o to 70 molar percent of ethylene and (b) about 30 to 70 molar percent of propylene and (II) from about 1 to 15 molar percent of a diene having isolated or nonconjugated double bonds, at least one of the double bonds of the diene being terminally located.

United States P'atent [72] Inventors Herbert F. Minter Pittsburgh;Richard D. Buckley, Stoneboro; Martin P. Seidel, Sharon, all of Pa.

[2!] Appl. No. 724,646

[22] Filed Jan. 12,1968

[45] Patented Dec. 7, 1971 [73 Assignee Westinghouse ElectricCorporation Pittsburgh, Pa.

Continuation-impart of application Ser. No. 355,382, Mar. 27, 1964, nowabandoned. This application Jan. 12, 1968, Ser. No. 724,646

[54] HIGH-VOLTAGE INSULATION AND INSULATED HIGH-VOLTAGE APPARATUS 9Claims, 1 Drawing Fig.

Insulating Materials for Design and Engineering Practice, J. Wiley &Sons( 1962) p. 45 l- 452 Primary E.raminer.lohn D. Welsh Attorneys-F.Shapoe and Alex Mich, Jr.

ABSTRACT: An admixture of i) from 55 to 70 percent by weight of hydratedalumina and 2) from 30 to 45 percent by weight of a petroleum oilextended ethylene-propylene-diene terpolymer, containing from 20 to 100phr. of oil, is molded about electrical apparatus to provide insulationfor components thereof. The hydrated alumina is advantageously finelydivided so that a major portion or substantially all particles are lessthan 2 microns. The terpolymer is derived from the reaction of anadmixture of(l) from about 85 to 99 molar percent of a mixture of (a)about 30 to 70 molar percent of ethylene and (b) about 30 to 70 molarpercent of propylene and (II) from about I to l5 molar percent of adiene having isolated or nonconjugated double bonds, at least one of thedouble bonds of the diene being terminally located.

PATENTEDUEB H971 3.626083 I. 20 9 I. l9 I2 I x V/ ///////,g //////4WITNESSES INVENTORS s Richard D. Buckley, H rt F. Minter inPS ATTORNEYHIGH-VOLTAGE INSULATION AND INSULATED HIGH- VOLTAGE APPARATUS CROSSREFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of application Ser. No. 355,382, now abandoned,filed Mar. 27, 1964.

BACKGROUND OF THE INVENTION This invention relates, in general, toelectrical apparatus having molded organic insulation which willwithstand exposure for prolonged periods to high-voltage stresses andcontaminating atmospheres. More particularly, this invention relates tominimizing the failure of electrical apparatus due to degradation oforganic insulation by long-term exposure to high electrical potentials.

Electrical insulating materials must have properties suitable not onlyto permit prolonged service in various'environments but also to permitfabrication of the electrical equipment by convenient and economicalmethods. Most organic insulating materials are easily fabricated orshaped by convenient molding methods and techniques but have propertieswhich limit its use in electrical equipment and/or the environments towhich it may be exposed. Electrical equipment which has organicinsulation disposed between members having a difference in electricalpotential is not generally suitable for prolonged use in environmentswhich include high humidity, moisture, salt and dust and high electricalstress. Such insulation must have a high voltage endurance and must beresistant to arcing, tracking and the surface creepage resultingtherefrom.

Arcing and tracking on organic insulation will occur when theinsulation, under an applied electrical stress, is exposed to conditionssuch as high humidity, moisture, contaminating particles or combinationsthereof. The environmental conditions will lower the resistance of theinsulation sufiiciently to permit a current leakage or current flowtherethrough. With current flowing through the insulation, enough heatcan be generated to carbonize the organic insulation and form alowresistance conducting path which can, in turn, produce equipmentfailure. Arcs discharging across the surface of the insulation can alsoproduce carbonization, low-resistance conducting paths and alsoultimately produce premature electrical equipment failures. Thesefailures are principally surface failures although some surface erosionof the insulation may occur.

Prolonged exposure to high-voltage stresses may cause organic insulationto fail or degrade by a different mechanism. Organic insulation whichhas been subjected to high-voltage stresses for prolonged periods willerode or sometimes crack. This failure is distinguishable from theheretofore described arcing and tracking failures since it occurs in theinsulation matrix itself. It should be understood that in manyapplications either or both types of failures can occur. Materials whichexhibit a subsurface erosion or cracking type of failure when exposed tohigh voltages, including voltage values above corona starting levels,are said to have low voltage endurance properties.

It is a known practice to incorporate certain inorganic compounds intoorganic materials to make the materials resistance to the arcing andtracking failures. Hydrated alumina, for example, has been added tobutyl rubber, epoxy resins and polyester resins to improve theirresistance to arcing and tracking failures. The hydrated alumina tominimize this surface type of failure in butyl rubber, for example, sothat the failures occur because of subsurface erosion upon prolongedexposure to high voltages. The art is empirical and there are noreliable guides by which the utility or merit of any particularadmixture of materials can be predicted. Other properties of the organicinsulation may, for example, be unimproved or even degraded. Sinceeither type of insulation failure can result in a failure of theequipment it is associated with, minimization of both types of failurewill permit the fabrication of more reliable equipment. Moreover, theincorporation of large amounts of hydrated alumina into polyester andother resinous insulation and particularly intov unvulcanized butylrubber, creates admixtures that are difficult to mold and shape. Withadmixtures that have poor moldability, distortion of the electricalapparatus during the molding process, as for example the coil to coil orcoil to core spacing in transformers is a problem.

Generally, the use of substantial amounts of oil extenders to reduce theviscosity of elastomeric electrical insulating formulations is notacceptable since the cure properties and the electrical properties,e.g., arc and track resistance and high voltage endurance are adverselyafiected. The use of substantial amounts of oil extenders is, however,attractive from the standpoint of cost since the oils are less expensivethan the elastomeric gum. Use of oil extenders could also permit higherfiller concentrations for a given required viscosity, the viscositybeing a measure of the flowability and therefore moldability of theadmixture. The higher filler concentrations could also improve the flameresistance of the molded insulation but the adverse effects of oilextenders on the reliability of equipment performance and cure of theelastomer are such that the attractive features of oil extension havebeen heretofore sacrificed.

SUMMARY OF THE INVENTION Briefly, the present invention provides aneasily moldable admixture of an ethylene-propylene terpolymer, apetroleum oil extender and relatively large amounts of finely dividedhydrate alumina. This readily moldable insulating material may bedisposed between spaced electrically conductive members between whichhigh electrical potentials are developed and which are exposed tocontaminating atmospheres. The insulation will have a high voltageendurance under corona conditions, an improved flame resistance and willbe resistant to arcing and tracking failures.

It is therefore a primary object of the present invention to provideelectrical apparatus having insulation which may be exposed to highvoltages and contaminating atmospheres for prolonged periods because ofits arc and tracking resistance and its high voltage enduranceproperties.

Another object of this invention is to provide a novel admixture of anethylene-propylene-diene elastomer, an extending oil and a finelydivided hydrated alumina filler which may be easily molded or shaped andwhich, in its cured form, may be exposed to high voltages andcontaminating environments for prolonged periods without failure.

Further objects and advantages of the invention will become apparent asthe following description proceeds and features of novelty whichcharacterize the invention will be pointed out in particularity in theclaims annexed to and forming a part of this specification.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of theinvention, reference may be had to the accompanying drawing, in which:

The single FIGURE is a perspective view, partially sectioned, of ahigh-voltage transformer representing one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has now been discovered thatadmixtures of an ethylenepropylene-diene elastomer, extending oil andhydrated alumina may be employed as insulation for electrical apparatus,equipment and members, as for example transformers, and exposed tolong-term rigorous electrical stresses and environmental conditionswithout the failures attending the heretofore employed insulations.Referring now to the single FIGURE of the drawing, we have illustrated atransformer 10 encased in the molded insulation 11, in accordance withthe present invention. A core member 12 is composed of laminations of asuitable magnetic steel, as for example silicon steel. Spaced around theside legs 13 and 14 of the core member 12, are the secondary windings l5and 16 of the transformer. The

secondary windings are spaced from the legs of the core member and thatspace is filled with the insulation 11 of this invention.

The primary windings 17 and 18 of the transformer are spaced around thesecondary windings. Leads 19 and 20 extend from the insulation mass 11and constitute, respectively, the primary and secondary terminals forthe transformer. It is apparent that the insulation 11 of this inventionencases the entire structure of the transformer, fills the spacesbetween the secondary and primary coils and the space between thesecondary coils and the legs of the core member to form a void-freemass. It should also be apparent that viscous, difficult to moldadmixtures will affect component spacing that can affect the designcharacteristics and performance of the transformer. If desired, theentire unit can be supported on a base member 21.

In view of the presence of spaced conductors that are subjected todifferent electrical potentials, arcing may occur and the insulation maybe subject to degradation by tracking and creepage especially onexternal surfaces exposed to contaminating atmospheres. Because of thehigh electrical potentials within the described structure and betweenthe described terminals, the insulation is also subject to degradationfrom continued voltage stress. in the operation of such structures,especially if undesirable voids are present in the insulation, theinsulation will be subjected to degradation by high-voltage stressesfrequently giving rise to corona conditions.

To provide prolonged service under typical but adverse conditions, theimproved insulation 11, in accordance with this invention, is anadmixture of an ethylene-propylene-diene elastomer, an extending oil andfinely divided hydrated alumina. It should be understood that the termethylenepropylene-diene elastomer describes certain specific syntheticelastomers or gums produced by the interpolymerization of alkenes,having a terminal double bond, as for example ethylene and propylene,with a diene having isolated double bonds, one of the double bonds beingterminally located. Catalysts, generally known as coordinationcatalysts, are used in preparing the interpolymers. The terpolymer gumsor elastomers are derived from the reaction of an admixture comprising(1) from about 85 to 99 molar percent of a mixture of about (a) 30 to 70molar percent ofethylene and (b) about 30 to 70 percent of propylene and(2) from about 1 to molar percent of a diene having the isolated ornonconjugated double bonds, at least one of the double bonds of thediene being terminally located.

Other mono-olefins having terminal unsaturation may be included in themixture of ethylene and propylene in minor quantities withoutdeleterious efiects. Indeed, minor quantities of other mono-olefins arenormally present as impurities in ethylene-propylene mixtures.

The diene may be any aliphatic compound, straight, open chain, orcyclic, or mixtures of two or more, so long as the double bonds areisolated and at least one is terminally located. The requirement thatthe double bonds be isolated or nonconjugated establishes the minimumnumber of carbon atoms at five. The upper limit may be as high as carbonatoms, although it is preferable to employ dienes having the minimumnumber of carbons or thereabout, to produce a relatively short sidechain linking the ethylene-propylene units of the polymer. Suitabledienes are, for example, bicyclopentadiene, cyclooctadiene,1,4-pentadiene, 1,4-hexadiene, 1,7- octadiene, l, l 9-eicosadiene andthe like.

Methods for preparing these gums or elastomers are known to thoseskilled in the art and reference may be had, for example, to US. PAT.No. 2,933,480 for specific details and these details are incorporatedherein by reference. Examples of commercially availableethylene-propylene terpolymer gums, under proprietary names, include EPT3509 from the Enjay Chemical Company, NORDEL from E. l. duPont deNemours & Co. and ROYALENE 200 from Naugatuck Chemical Company.

An essential component in the insulating composition of this inventionis, of course, hydrated alumina. For the attainment of the necessarydegree of arc and track resistance, voltage endurance under coronaconditions and fiame resistance, from about 55m 70 percent of hydratedalumina, on a total weight basis should be employed. A distinctadvantage of the insulation of this invention is the possibility andease of including the hydrated alumina in larger proportions, than maybe included with other elastomeric materials where the use of extendingoils is severely limited or prohibited.

Although some improvement may be contemplated from any particulated orfinely divided hydrated alumina in the specified range of composition, apronounced increase in the arc and track resistance occurs when finelydivided hydrated alumina having a major portion of the particles lessthan 2 microns in size is employed. The ultimate resistance to arcingand tracking will, for example, be attained if all of the hydratedalumina particles are less than 2 microns in size as determined byelectron microscope. Using particles that are substantially larger thanabout 10 microns in the admixture will result in a lower degree of arcand track resistance. That may not pass the 8. l-watt energy level testsdescribed hereinbelow. I

The extender oils are employed in amounts ranging from 20 to lOO phr. ofelastomeric terpolymer to produce easily molded stock without anundesirable degradation of the arc and track resistance or the improvedvoltage endurance, corona resistance and other properties attending thisinvention. Indeed, the inclusion of the extender oil permits relativelyhigher loadings of hydrated alumina which provide an increase in flameresistance. It should be again emphasized that the foregoing substantialamounts of oil cannot be employed with the butyl gum or otherelastomeric materials that might be employed as electrical insulation.To our knowledge, none of the heretofore-employed elastomers could becompounded with the substantial amounts of hydrated alumina and oil toprovide satisfactory insulating compositions. It is only the describedelastomeric terpolymer that will provide a combination suitable forelectrical apparatus such as transformers.

The described elastomeric terpolymer is compatible with most hydrocarbonor petroleum oils, especially saturated oils of low polarity such asparaffinic and naphthenic types, and as noted heretofore they may beadded in amounts from l0 phr. to as high as the amount of terpolymer gumitself, on a weight basis. Examples of suitable proprietary petroleumoils include Necton 60, Circosol 2XH, Flexon 765, Shellflex 790 and SunOil 5150. Petroleum or hydrocarbon oils having a viscosity of to 5,000Saybolt Universal seconds at 100 F. may be employed as extender oils inthe insulating admixtures of this invention. A suitable paraffinic oil,employed in the examples hereinbelow, contained 78.3 percent paraffiniccarbons, 20.8 percent aromatic carbons and 0.9 percent polar compounds.The viscosity of the oil was 490 Saybolt Universal seconds at 100 F.

Another particularly suitable extending oil contains 4 percent aromaticcarbons, 23 percent naphthenic carbons and 73 percent paraffiniccarbons. The preferred range of petroleum oil extender is from 50 to 65phr.

In addition to the essential hydrated alumina, small amounts of otherfillers may be employed in combination with the elastomeric terpolymer.Small quantities of carbon black, in the order of about 1 to 5 phr., maybe employed as a colorant and to absorb ultraviolet energy and limit thedegrading effects of light to a thin surface layer. Other fillers, toimprove other properties may be included, in accordance with principlesknown in the art, if desired. For example, about 10 percent by weight offinely divided clay, calcium silicate, talc or silica may be included asa filler to improve the molding characteristics and ultimate physicalproperties of the vulcanized admixtures.

Metal oxides such as magnesium oxide or zinc oxide, in amounts rangingfrom about 3 to 20 phr., may be employed to produce improvements inphysical and electrical stability.

Small amounts of plasticizer in amounts up to about phr., may be addedto further improve the moldability of the stock. Examples of suitableplasticizers include fatty acids, metal salts and esters of fatty acids,vegetable oils and petroleum waxes, for instance, calcium stearate.

Vulcanizable silicones and certain dioximes such as p-quinonedioxime anddibenzoyl-p-quiononedioxime may be added to improve the power factor andwater resistance.

Curing systems for compositions or formulations that include theethylene-propylene terpolymer gum are preferably the conventional sulfursystems known in the art. A low-sulfur vulcanizing system is recommendedfor maximum insulating values. Peroxide and phenolic resin systems mayalso be employed.

The insulating compositions of this invention may be described asadmixtures of (A) hydrated alumina and other inorganic fillers and (B) apetroleum oil extended ethylenepropylene-diene elastomer containing fromto I00 phr. of petroleum oil. The (B) petroleum oil extended elastomeris intended to cover formulations of ethylene-propylene-diene elastomer,described heretofore, together with the specified amount of extendingoil plasticizers, vulcanizing agents, accelerators and other elastomercompounding additives those skilled in the art would ordinarily includein compounded rubber mixtures. In accordance with these principles,these insulating compositions may be described as admixtures, on a totalweight basis, of (A) from about 55 to 70 percent of particulatedhydrated alumina and (B) from about 30 to 45 percent of a petroleum oilextended ethylene-propylene-diene elastomer containing from 20 to 100phr. of petroleum oil.

The invention will be further described in the following examples of amolded transformer. The details are illustrative of the invention andshould not be considered limiting.

One hundred parts, by weight, of an ethylene-propylenediene gum derivedfrom an admixture of about 60 molar percent of ethylene, 39 molarpercent of propylene and 1 molar percent of bicyclopentadiene is addedto a Banbury mixer together with l phr. (parts per hundred) ofdibenzoyl-p-quinonedioxime and mixed until a temperature of about 340 to380 F. is reached. The mixed material is discharged and cooled. Thecooled material together with 341 phr. of hydrated alumina, 3 phr. ofcarbon black, 5 phr. of zinc oxide, 2 phr. of stearic acid, 56 phr. ofthe heretofore-described paraffmic petroleum oil, 5 phr. zinc stearate,4 phr. of petroleum wax and 4 phr. of a vulcanizable silicone is mixeduntil a temperature of about 220 to 300 F. is reached in order to driveoff moisture in the fillers. The mix is discharged and cooled. Togetherwith l phr. of sulfur, l phr. of zinc dimethyldithiocarbamate, l phr. oftetramethylthiuram disulfide, I phr. of telluriumdiethylodithiocarbamate and l phr. of benzothiazyl disulfide the cooledmix is recharged, mixed to a temperature of about 180 F. and dumped.This unvulcanized stock is then employed in molding transformers inaccordance with this invention. It should be noted that the pretreatmentof the gum with dibenzoyl-p-quinonedioxime considerably reduces totalmixing time.

A mold cavity heated to a temperature of about 300320 F. is sprayed witha release agent. A portion of the unvulcanized stock is injected intothe mold, where it remains for about 30 minutes and is fully cured.Compared to the heretofore employed butyl rubber formulations, theformulations of this invention require less molding force and thusproduce less distortion of transformer assemblies. If the admixture isdegassed no internal or surface flaws will be apparent. Porefree crosssections may be attained in one-third less mold cure times as comparedto butyl rubber formulations. The low viscosity of the compounds of thisinvention at room temperature has permitted molding of units without thedielectric preheating of stock, a common practice in the processing ofthe butyl stock heretofore employed.

Additional examples of admixtures which are suitable in accordance withthis invention are presented in convenient form in table I. The amountsof materials are expressed in parts per hundred ofgum or elastomer (phr.

Table I Material A B C D E Ethylene-propylene- I00 I00 I00 100 diene gumDibenloyl-p- 1.0 1.0 L0 1.0 1.0 quinonedioxime Hydrated alumina 235 223M3 J4l I82 Petroleum oil 56 20 40 56 20 Carbon black 3.0 3.0 3.0 3.0 3.0Stearic acid 2.0 2.0 2.0 2.0 2.0 Zinc steurute 5.0 5.0 5.0 5.0 5.0Vulcunizahle silicone 4.0 4.0 4.0 4.0 4.0 Petroleum wux 4.0 4.0 4.0 4.04.0 Zinc oxide 5.0 5.0 5.0 5.0 5.0 Sulfur 1.0 L0 L0 L0 L0 Zincdimethyldithiocurbumate L0 L0 (3.0) (3.0) (3.0) MercaptobcnzothiazoleZinc benmthiazyl L0 L0 L0 sulfide Tetramethylthiuram L0 L0 disulfideTellurium diethyl- L0 1.0 dithiocarbamate Benzothiazyl disulfide L0 [.0

Each of these formulations were molded and produced satisfactorymembers.

As noted hereinabove, organic insulation employed in electricalapparatus exemplified by'the transformed illustrated in FIG. I, issubject to arcing and tracking and failure from exposure to high-voltagestress under corona conditions and in the presence of ozone. To evaluatethe arc and track resistance of the insulation of this invention,samples of an admixture of ethylene-propylene terpolymer rubber andhydrated alumina in varying proportions, were subjected to a testdescribed by Mandelcorn and Dakin in an article entitled Wet SurfaceTracking of Insulation; A Differential Test With Controlled ShortDischarges to a Water Electrode, AIEE Transactions, Part III (PowerApparatus and Systems), Volume 81, I962,

page 291.

One inch square samples are subjected to a metal-to-liquid electricdischarge continuously for 1 minute. The liquid is a 1 percent, byweight, ammonium chloride solution in water and contains 0.1 percent ofa wetting agent (Aerosol OT). Immediately after discharge, in this caseat a power level of 8.l watts, the surface is tested successively at 500and L000 volts and must not reveal a conducting path at these levels.Data in this test have been correlated by Mandelcorn and Dakin withthose of the conventional Dust and Fog Test (ASTM Method D-2l32-62T),and samples passing the 8. l-watt test correspond to a period greaterthan 500 hours in the ASTM test.

Sample stock for the foregoing test were prepared in accordance with theheretofore-described formulations. All samples employing hydratedalumina where essentially all of the particles were less than 2 micronspassed the high-level 8. l-watt test. Less consistent results wereobtained with hydrated alumina having larger particle sizes.

At page 1033 of the AIEE Transactions on Power Apparatus and Systems,No. 69, Dec. 9, 1963, Hewitt and Dakin, in an article entitled VoltageEndurance Tests of Insulating Materials" describe an acceleratedprocedure for testing the voltage endurance of insulating materials athigh frequencies. Employing the equipment and procedures describedtherein, the voltage endurance of void-free samples of alumina-filledbutyl rubber and alumina-filled ethylene-propylene-diene terpolymer wascompared at various concentrations of hydrated alumina.

Sample stocks were prepared by methods similar to that employed in thepreparation of the sample stock for the arc and track resistance tests.Void-free samples, 5 5 0.070 inches were molded employing both butylrubber and ethylene-porpylene-diene terpolymer rubber, each with varyingamounts of hydrated alumina from about 50 to 60 percent by weight. The

samples were machined to a thickness of about 60 mils in the test area,placed between a cylindrical self-aligning stainless steel upperelectrode about one-half inch in diameter and a cylindrical stainlesssteel lower electrode having a minimum diameter of l inches. A force of0.25 to 0.4 pounds was applied to the upper electrode. The samples weresubjected to various constant voltages, from about 150 to 240 volts rmsper mil at 1,500 cycles per second.

Samples formulated with the ethylene-propylene-diene terpolymercontaining and 56 phr. of a paraffinic oil had an equivalent 60 cyclelife far exceeding those formulated from butyl rubber stock containing 4phr. of oil. For example, ethylene-propylene-diene terpolymer samplescontaining about 55 percent, by weight, of hydrated alumina and testedat 163 volts RMS/mil. had an equivalent 60-cycle life of about 83,500hours whereas butyl rubber samples containing about 58 percent ofhydrated alumina, by weight, had an equivalent 60-cycle life of only[7,500 hours. It should be noted that all of the test voltages were highenough to exceed the corona threshold voltage. Whether the test chamberwas maintained in dry condition or at a relative humidity of 50 percent,whether the electrode edges were round or flat, a distinct surpriseimproved endurance to high voltage by the ethylene propylene terpolymersamples was apparent at all voltages and filler concentrations.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, modifications theretowill readily occur to those skilled in the art. For example, theethylene-propylene terpolymer gum can be blended or covulcanized withminor proportions of other elastomeric gums which are cross-linked bythe particular type of cure system being employed to vulcanize theethylene-propylene terpolymer rubber, although the same high degree ofvoltage endurance may not be attained. It is not desired, therefore,that the invention be limited to the specific arrangements shown anddescribed and it is intended to cover in the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

We claim:

I. In electrical apparatus having spaced electrically conductive membersbetween which an electric potential is developed, a cured organic baseinsulating material having improved voltage endurance propertiesdisposed between said members, the insulating material consistingessentially of a vulcanized mixture of (A) from about 55 to 70 percent,by weight, of particulated hydrated alumina and (B) from about 30 to 45percent, by weight, of a petroleum oil extended ethylene-propylene-dieneterpolymer consisting essentially of (I) from about 85 to 99 molarpercent of a mixture of (a) about 30 to 70 molar percent of ethylene and(b) about 30 to 70 percent of propylene and (2) from about I to l5 molarpercent of a diene having isolated double bonds, at least one of thedouble bonds being terminally located, and containing from 20 to 100phr. of the petroleum oil.

2. The apparatus of claim 1 wherein a major portion of said hydratedalumina has a particle size less than 2 microns.

3. The apparatus of claim 1 wherein the particulated hydrated aluminahas substantially all of its particles of a particle size less than 2microns and (B) the petroleum oil content is from 50 to 65 phr. ofethylene-propylene-diene terpolymer.

4. The apparatus of claim 1 wherein said spaced electrically conductivemembers are elements of a transformer and the insulating material ismolded to support and largely surround the electrically conductivemembers.

5. A cured organic base electrical insulating composition suitable forprolonged exposure to arcing, tracking and highvoltage stresses undercorona conditions consisting essentially of an intimate admixture of (A)from about 55 to 70 percent of a particulated hydrated alumina and (B)from about 30 to 45 percent, by weight, of a petroleum oil extendedethylenepropylene terpolymer consisting essentially of l fromabout to 99molar percent of a mixture of (a) about 30 to 70 molar percent ofethylene and (b) about 30 to 70 percent of propylene and (2) from about1 to 15 molar percent of a diene having isolated double bonds, at leastone of the double bonds being terminally located, and containing from 20to 100 phr. of the petroleum oil.

6. The composition of claim 5 wherein a major portion of the hydratedalumina has a particle size less than 2 microns.

7. The composition of claim 5 wherein the particle size of substantiallyall of the hydrated alumina is less than 2 microns.

8. The composition of claim 5 wherein the petroleum oil is within therange of 50 to 65 phr.

9. In electrical apparatus having spaced electrically conductive membersbetween which an electrical potential is developed, a cured organicbase-insulating material having improved voltage endurance propertiesdisposed between said members, the improvement comprising the insulatingmaterial consisting essentially of a vulcanized mixture of (A) fromabout 55 to 70 percent, by weight, of particulated hydrated alumina and(B) from about 30 to 45 percent, by weight, of a petroleum oil extendedethylene-propylene-diene terpolymer consisting essentially of l fromabout 85 to 99 molar percent ofa mixture of (a) about 30 to 70 molarpercent of ethylene and (b) about 30 to 70 percent of propylene and (2)from about I to 15 molar percent of a diene having isolated doublebonds, at least one of the double bonds being terminally located, andcontaining from 20 to 100 phr. of the petroleum oil.

i i l

2. The apparatus of claim 1 wherein a major portion of said hydratedalumina has a particle size less than 2 microns.
 3. The apparatus ofclaim 1 wherein the particulated hydrated alumina has substantially allof its particles of a particle size less than 2 microns and (B) thepetroleum oil content is from 50 to 65 phr. of ethylene-propylene-dieneterpolymer.
 4. The apparatus of claim 1 wherein said spaced electricallyconductive members are elements of a transformer and the insulatingmaterial is molded to support and largely surround the electricallyconductive members.
 5. A cured organic base electrical insulatingcomposition suitable for prolonged exposure to arcing, tracking andhigh-voltage stresses under corona conditions consisting essentially ofan intimate admixture of (A) from about 55 to 70 percent of aparticulated hydrated alumina and (B) from about 30 to 45 percent, byweight, of a petroleum oil extended ethylene-propylene terpolymerconsisting essentially of (1) from about 85 to 99 molar percent of amixture of (a) about 30 to 70 molar percent of ethylene and (b) about 30to 70 percent of propylene and (2) from about 1 to 15 molar percent of adiene having isolated double bonds, at least one of the double bondsbeing terminally located, and containing from 20 to 100 phr. of thepetroleum oil.
 6. The composition of claim 5 wherein a major portion ofthe hydrated alumina has a particle size less than 2 microns.
 7. Thecomposition of claim 5 wherein the particle size of substantially all ofthe hydrated alumina is less than 2 microns.
 8. The composition of claim5 wherein the petroleum oil is within the range of 50 to 65 phr.
 9. Inelectrical apparatus having spaced electrically conductive membersbetween which an electrical potential is developed, a cured organicbase-insulating material having improved voltage endurance propertiesdisposed between said members, the improvement comprising the insulatingmaterial consisting essentially of a vulcanized mixture of (A) fromabout 55 to 70 percent, by weight, of particulated hydrated alumina and(B) from about 30 to 45 percent, by weight, of a petroleum oil extendedethylene-propylene-diene terpolymer consisting essentially of (1) fromabout 85 to 99 molar percent of a mixture of (a) about 30 to 70 molarpercent of ethylene and (b) about 30 to 70 percent of propylene and (2)from about 1 to 15 molar percent of a diene having isolated doublebonds, at least one of the double bonds being terminally located, andcontaining from 20 to 100 phr. of the petroleum oil.